The present invention relates to a panel type X-ray solid detector, and especially, the invention relates to a two-dimensional array type radiation detector used for a multi-slice X-ray CT apparatus or a cone beam X-ray CT apparatus.
In an X-ray CT apparatus, X-ray is irradiated from an X-ray tube, and concentrated into a fan-shaped X-ray beam by a collimator at a radiation port, and the X-ray tube, the collimator in an arc shape disposed to face the X-ray tube and the detector are rotated around a subject to be detected. Then, the detector captures information of X-ray which has passed through the subject so that signals thereof are processed by a computer to obtain the X-ray tomograph image of the detected subject.
The detector has a structure such that approximately 500 to 1000 channels of X-ray detecting elements, each of which is formed of a scintillator element for converting X-ray into light and a photodiode for detecting the light converted by the scintillator element to output as the electric signal, are arranged in an arc shape around the X-ray tube.
From the viewpoint of the mechanical arrangement upon manufacturing, 8 to 30 pieces of a combination of the scintillator and photodiode adhered optically are arranged on the base plate to constitute one module, and the above detector modules are arranged continuously on a circumference in an approximately arc shape and combined with the collimator to form the solid detector for CT.
The conventional radiation detector is structured as described above. Since only one slice of the tomograph image is obtained in the detector arranged one-dimensionally in a channel direction (a direction perpendicular to a body axis of the detected subject), it is required to scan many times in order to obtain the multi-layered tomograph images. In order to obtain the many slice data at once, there has been considered a two-dimensional array type radiation detector in which X-ray detecting elements are arranged not only in the channel direction but also in a slice direction (a body axis direction of the detected subject) perpendicular to the channel direction, and a detector, which is used together with a cone beam X-ray tube to collect multi-slice data with a minute slice pitch in a short time, has been expected.
This two-dimensional array type radiation detector is formed of an X-ray converting film which normally converts X-ray into light; photodiode arrays arranged in a matrix form right under the x-ray converting film; and switching elements connected to the respective photodiode arrays. There are two types of the two-dimensional array type radiation detector. Namely, a first type is a detector in which the switching elements are sequentially turned on after irradiation of the X-ray, so that signal charges stored in the respective pixels are read out to form the X-ray image. A second type is a detector which has a radiation sensor array formed of a converting film directly outputting a charge signal corresponding to an incident amount by responding to the radiation, and switching elements connected to electrodes arranged in a matrix form arranged right under the radiation sensor array, wherein the respective switching elements are sequentially turned on at the time of irradiation, so that the signal charges stored in the respective pixels are read out to form the X-ray image. Here, the first type will be explained.
FIG. 4 shows a detection circuit of the first type of the two-dimensional array type radiation detector. In the detector, scintillators 11 for converting X-ray into light are formed uniformly on a front surface of the detector, and on a rear surface of each scintillator, photoelectric conversion elements 1, such as photodiodes, which convert light into electric signal, are arranged in an array in order vertically and horizontally. Then, switching elements, such as FETs 2, are formed to form pairs with the photoelectric conversion elements 1, and terminals of sources 5 of the respective FETs 2 are connected to the photoelectric conversion elements 1. Terminals of gates 3 of the respective FETs 2 are connected to gate bus lines in a horizontal direction, and terminals of drains 4 of the respective FETs 2 are connected to data bus lines 7 in a vertical direction. A gate driver section 8 and a data collecting section 9 are controlled by a control section 10, and pulse signals from the gate driver section 8 are outputted to the gates 3 of the FETs 2, sequentially from the upper side to the lower side through the gate bus lines 6 formed in the horizontal direction. Picture charge signals stored in the photoelectric conversion elements 1 are read out from the sources 5 to the drains 4, and taken into the data collecting section 9 from the data bus lines 7 formed in the vertical direction. Then, the picture data signals which are taken are outputted to an outside from the data collecting section 9.
FIG. 5 shows a sectional structure of the two-dimensional array type radiation detector. In the detector, the scintillator 11, photoelectric conversion elements 1 (a-Si:H PDA or the like) and switching elements (a-Si:H FET 2 or the like) are formed on a base plate 13. The gate bus lines 6 are formed on the base plate 13, and the electrodes of the gates 3 of the FETs 2 are laminated on the gate bus lines 6. Then, the FETs 2 as the switching elements are formed orderly side by side. Also, the data bus lines 7 on an insulation 16 are laminated on the electrodes of the drains 4 of the FETs 2. And, the gate bus lines 6 situated in the lower layer and the data bus lines 7 situated in the upper layer are arranged perpendicular to each other with the insulation therebetween. Then, the photoelectric conversion elements 1 are formed orderly side by side on the electrodes of the sources 5 of the FETs 2. The scintillator 11 is formed on the entire upper surfaces of the photoelectric conversion elements.
The conventional two-dimensional array type radiation detector is structured as described above. In case the two-dimensional array type radiation detector is applied to the cone beam X-ray CT apparatus, a matrix size of the detector ranges from 512×512 to 2,048×2,048, and an outside size of the detector is of a square with one side of 200 mm to 300 mm, so that the size of the detector can not respond to the size required in the cone beam X-ray CT apparatus, for example, 1,000 mm×200 mm. Also, even if the four detectors respectively having a size of 300 mm×300 mm are connected to have the size of 1,200 mm×300 mm, the gate bus lines 6 are arranged perpendicular to the data bus lines 7 such that the gate driver section 8 for the horizontal scanning occupies one side and the data collecting section 9 for reading data occupies another side, or in some cases, the gate driver section 8 and the data collecting section 9 occupy all the four sides. Thus, even if the detectors are arranged side by side in a traverse direction, since the electronic components, such as scanning circuits, are mounted in connection portions thereof, a large dead space is formed. Thus, the above arrangement is not practical.
The present invention has been made in view of the foregoing, and an object of the invention is to provide a two-dimensional array type radiation detector, which has a large detecting surface with a two-dimensional array type required for the cone beam X-ray CT apparatus.
Further objects and advantages of the invention will be apparent from the following description of the invention.